The present invention relates generally to optical fibers. More specifically, the present invention pertains to methods of aligning optical fibers. Optical fibers incorporate a core designed to transmit light along its length, and cladding around the core along its entire length. Like the core, the cladding transmits light that strikes its end, but it does not transmit light to or from the core. The present invention is particularly, though not exclusively, useful for aligning an optical fiber with a converging light beam by using sensors that sense the amount of light leaking from the cladding into a ferrule around the fiber.
Over the past several decades, the use of optical fibers, or fiber optics, to transmit information on a light beam have become increasingly popular. In fact, much of the information which is transmitted today is done over optical fibers. Alignment of a light beam into an optical fiber typically requires optical feedback to correct positioning of either the fiber or light beam. In some cases, particularly where single-mode (SM) fiber is used, continual monitoring of optical feedback is necessary to maintain proper alignment of the light beam with the fiber.
An object of the present invention is to provide an apparatus capable of aligning a light beam with a fiber so that the focal point of the light beam aligns with the core of the fiber, responsive to the amount of light leaking from the cladding of the fiber into a ferrule surrounding the fiber.
When a light beam is aligned with a fiber, there is often a portion of the light beam that falls outside the core. This light, which falls into the cladding region of the fiber, may be used as an error signal. This invention removes the light from the cladding and measures the intensity of this light to determine the positional error between the fiber and the incident light beam.
Where a single beam is to be focused into a fiber, the light falling into the fiber""s cladding can be used to focus the beam into the fiber, provided that the light in the cladding is removed from the cladding before the light scatters into other modes and loses its directional information. By positioning the fiber within a glass ferrule, the light may be captured and removed from the cladding without the introduction of confusing or subsequent reflections. To further minimize any reflections, the ferrule may be index-matched with the fiber.
While current methods may use the light falling into the cladding as an indication of positional error, these methods can measure only the magnitude, not the direction, of the error. In those systems, the control loop must then use a dithering technique to determine direction, slowing down the response time of any positional feedback and correction efforts.
The light beam entering a fiber is usually gaussian in profile, being generated by another fiber or from a laser. Consequently, most of the light is concentrated in the center of the beam, which is to be focused with an input lens and carefully aligned to the fiber core. In ordinary systems, any light falling into the cladding will propagate distances of perhaps a meter, undergoing many reflections and losing any information related to the positional errors between the light beam and the fiber core.
Cladding and index-matching mode strippers are currently available to remove the light from the cladding. These strippers incorporate the bending the fiber in a serpentine fashion which causes the light to leak out of the fiber. However, using these devices removes the positional error which is essential in an efficient position feedback system.
The present invention uses a large diameter, rigid fiber ferrule which is index matched to the fiber cladding so that multiple reflections do not occur. Since the light that misses the core and strikes the fiber cladding can be detected before it gets reflected, the measurement of this light can be used as an error signal in a position correction fast control loop. The light can be extracted before it undergoes multiple reflections by either using a short glass ferrule, so the light exits out the bottom, or by drilling holes into the side of the ferrule and adding a scattering center to intercept some of the light.
A main advantage of-the-present invention is that only the light beam itself is required for proper positioning on the fiber core, and any light striking the fiber cladding generates an error signal that is very well correlated with the light through the core. This error signal may be simply used to provide position feedback correction to maximize the light received by the fiber core.
An important characteristic of optical fibers addressed by the present invention is the concentricity of the fiber core with respect to the outside diameter of the fiber. Typically, the fiber core is concentric to the outside diameter within 0.5 microns; this is the required concentricity for standard connectors. Using the light leaking from the fiber cladding, the present invention achieves this degree of accuracy in the alignment of the converging light beam with the fiber core.